This invention relates to vacuum gauges and more particularly to ionization gauges for use in the ultra-high vacuum range.
In known ionization gauges, the number of positive ions formed within the gauge, in a gas susceptible to ionization by election impact, is directly proportional to the molecular concentration of the gas. Known ionization gauges typically comprise a source of electrons (cathode), an accelerating electrode (anode) to maintain electron current, and a collecting electrode (collector) to collect the ions formed by electron impact in the gas. While ion formation is not believed theoretically to have a low pressure limitation, one of the more serious practical barriers to useful ultra-high vacuum measurement is the production of undesirable extraneous currents in the gauge which are independent of gas pressure.
The undesirable extraneous currents principally result from the so-called X-ray effect. Bombardment of the anode by electrons produces soft X-rays. The soft X-rays impinge the collector, thereby producing a photo-electron current which adds to the ion current in the collector. As photo-electron current and the ion current are not distinguishable from one another, the photo-electron current establishes a lowest practical limit beyond which meaningful ion current measurement cannot be had.
One known type of ionization gauge incorporates a fine wire as the collector. The anode is a grid-like structure. Such an apparatus is disclosed in U.S. Pat. No. 2,605,431, issued July 29, 1952 to Bayard. Known ionization gauges incorporating thin wire collectors are suitable for measuring pressures as low as 3.times.10.sup.-10 Torr with an open ended grid volume, and 2.times.10.sup.-11 Torr with a close ended grid volume. Measurements of even lower pressures is desirable, however.
It is known to reduce the diameter of a collector to less than approximately 0.002 inches for decreasing interception of the X-ray flux. Because the ion current decreases approximately proportionally, however, the lowest measurable pressure limit of an ionization gauge cannot be extended by merely reducing collector diameter.
It is known to achieve ultra-high vacuum measurements with extremely small diameter ion collectors of approximately 4 microns (0.00016 inches) by applying an unusually high ion collection voltage between the anode and collector. Such an apparatus is disclosed in U.S. Pat. No. 3,253,183, issued May 24, 1966 to Van Oostrom. Although the X-ray flux impinging the collector is reduced, the disadvantages include the need for an abnormally high ion collection voltage and supporting structures for both ends of the collector.
It is also known to reduce the X-ray flux impinging the collector by shortening the collector length. The disadvantage experienced in the art by such an approach, however, is to seriously decrease the percentage of positive ions collected.
It is also known to completely withdraw the ion collector from the grid volume in which positive ions are formed. Ion extraction may depend on field penetration, as in U.S. Pat. No. 3,463,956, issued Aug. 26, 1969 to Groszkowski. Alternatively ion extraction may depend on the application of a separate accelerating voltage to one or more additional electrodes. An example of a device based on the latter principle is found in U.S. Pat. No. 3,465,189, issued Sept. 2, 1969 to Redhead. The disadvantage of such prior art ion extractor gauges is the need for a separate accelerating voltage and one or more additional electrodes if a reasonable gauge sensitivity is to be achieved. The inclusion of an accelerating voltage and additional electrode, however, increases the complexity of construction and difficulty of use.